1. Field of the Invention
The invention relates to a LINC transmitter and, in particular, to a multilevel LINC transmitter.
2. Description of the Related Art
To prolong battery life of mobile handset devices, power efficiency demands from wireless mobile communication systems have become more important. Specifically, a transceiver's most power hungry device is a power amplifier which has nonlinear characteristics. Meanwhile, modulation of non-constant-envelope signals demands high linearity from a power amplifier. As a result, there is a trade off between linearity and power efficiency in a wireless transmitter.
Various PA linearization techniques have been adopted to improve linearity and power efficiency of wireless transmitters. Linear amplification with nonlinear components (LINC) is a transmitter architecture which increases linearity and power efficiency of a wireless transmitter. Due to accurate signal processing and insensitivity to process variations, a digital LINC architecture is more suitable for modern process technologies.
FIG. 1 is a block diagram of a conventional LINC architecture. Referring to FIG. 1, an input signal S(t) of the LINC 100 is a varying envelope signal. A signal separator 110 receives and divides the input signal S(t) into two constant-envelope signals S1 and S2. Subsequently, two power amplifiers PA1 and PA2 amplify the constant-envelope signals S1 and S2, respectively. Since a nonlinear power amplifier can amplify a constant-envelope signal linearly, two power efficient nonlinear power amplifiers are used in such architecture. Finally, the two amplified signals are combined by a power combiner 120. Thus, a linearly amplified signal is obtained at an output of the power combiner 120.
The input of the LINC system is a varying-envelope signal S(t),S(t)=A(t)·ejφ(t) wherein A(t) denotes the signal envelope and φ(t) is the signal phase. In the phasor diagram shown in FIG. 2A, the varying-envelope signal S(t) is split into a set of constant-envelope signals, S1(t) and S2(t),
                              S          ⁡                      (            t            )                          =                              1            2                    ⁡                      [                                                            S                  1                                ⁡                                  (                  t                  )                                            +                                                S                  2                                ⁡                                  (                  t                  )                                                      ]                                                  =                              1            2                    ⁢                                    r              0                        ⁡                          [                                                ⅇ                                      j                    ⁡                                          (                                                                        φ                          ⁡                                                      (                            t                            )                                                                          +                                                  θ                          ⁡                                                      (                            t                            )                                                                                              )                                                                      +                                  ⅇ                                      j                    ⁡                                          (                                                                        φ                          ⁡                                                      (                            t                            )                                                                          -                                                  θ                          ⁡                                                      (                            t                            )                                                                                              )                                                                                  ]                                          And an out-phasing angle θ(t) is expressed as
      θ    ⁡          (      t      )        =            cos              -        1              ⁡          (                        A          ⁡                      (            t            )                                    r          0                    )      Both S1(t) and S2(t) are on a circle with a radius r0. In a conventional LINC transmitter, r0 is a constant scale factor predefined by a system designer. Because input range of an inverse cosine function is [−1, 1], selection of r0 is required to satisfy the formula:r0≧max(A(t))
FIG. 2B illustrates the signals after amplification. The amplified signals are expressed as G·S1(t) and G·S2(t), where G is voltage gain of the power amplifiers. The two amplified signals are combined by a power combiner to obtain a signal √{square root over (2)}G·S(t) which is a linear amplification of the input signal S(t). Because of the out-phasing technique, LINC achieves linear amplification with two power efficient nonlinear power amplifiers.